Laetoli Footprints: The 3.66-Million-Year-Old Hominin Trackway

The Laetoli footprints are a trail of fossilised footprints left by early hominins in volcanic ash about 3.66 million years ago in what is now northern Tanzania. Found by Mary Leakey’s team in 1978, they are the oldest clear evidence of creatures walking upright with a human-like stride. They showed that our ancestors were walking on two legs more than a million years before their brains began to grow significantly larger.

Background: Laetoli and the Leakeys

Laetoli lies on the edge of the Serengeti Plains, about 45 kilometres south of Olduvai Gorge, inside today’s Ngorongoro Conservation Area. Its name comes from the Maasai word for a red lily that grows in the area. Mary and Louis Leakey first visited in 1935, and a German expedition led by Ludwig Kohl-Larsen collected fossils there in the late 1930s.

Mary Leakey began a new project at Laetoli in 1974. Her team found hominin jaws and teeth, including the lower jaw LH 4, which in 1978 became the type specimen of the new species Australopithecus afarensis, alongside fossils from Hadar in Ethiopia such as Lucy.

How the footprints were made

Around 3.66 million years ago a nearby volcano, probably Sadiman about 20 kilometres to the east, erupted repeatedly and covered the landscape in fine ash rich in carbonatite, an unusual volcanic material. Light seasonal rain dampened the ash, which took impressions like wet cement as animals walked across it. Even raindrop marks were preserved.

As the ash dried, the carbonate in it set hard. Further falls of ash buried the surface, sealing in the prints. Geologists call this layer the Footprint Tuff. Over millions of years erosion brought parts of it back to the surface.

Discovery: from elephant dung to footprints

In 1976, according to a well-known account, several visiting scientists including Andrew Hill were throwing dried elephant dung at each other in play. Diving for cover, Hill noticed animal footprints in the hard surface. The team went on to record thousands of tracks of animals such as hares, antelopes, giraffes, rhinoceroses, elephants, guinea fowl and an extinct three-toed horse.

Some possible hominin prints were found at Site A in 1976, but they were unusual and their identity was disputed. In 1978 the American geochemist Paul Abell noticed what looked like a human heel print at another locality, Site G. Excavation in 1978 and 1979 exposed a double trail about 27 metres long containing around 70 hominin footprints.

What the trail shows

Two trails run side by side, heading roughly north. One, called G1, was made by a smaller individual. The other was made by a larger individual, G2, and a third, G3, appears to have stepped into G2’s prints, making that trail harder to read. Height estimates based on foot length and stride suggest the walkers were very roughly 1.2 to 1.5 metres tall.

The prints show features found in modern human footprints:

• a deep heel strike, showing the heel hit the ground first;
• a big toe in line with the other toes, not splayed out like an ape’s grasping toe;
• signs of an arch along the foot;
• a weight transfer from heel to the ball of the foot and then the big toe.

Experiments in 2010 led by David Raichlen compared the prints with those of people walking normally and with bent knees and hips. They concluded the Laetoli walkers moved with an extended-leg gait similar to modern humans, not with the crouched walk of a chimpanzee standing upright.

Who made them

No bones were found in the trackway, so its makers cannot be identified with certainty. Most researchers attribute the prints to Australopithecus afarensis, because it is the only hominin species known from Laetoli in the same period and its skeleton suggests a similar foot. Some scientists have argued that the prints look more modern than the known afarensis foot, but this remains a minority view.

In 2015 Tanzanian and Italian researchers found prints of two more individuals at Site S, about 150 metres from Site G, published in 2016. One, known as S1, appears to have been considerably larger than the Site G walkers, perhaps around 1.65 metres tall, suggesting marked size differences, possibly between males and females.

In 2021 a re-excavation of the Site A prints led by Ellison McNutt concluded that they too were made by a bipedal hominin, but one walking differently from the Site G individuals. If correct, this suggests that two kinds of hominin may have lived at Laetoli at the same time.

Conservation

After excavation the Site G trail was reburied in 1979 to protect it. By the early 1990s acacia tree roots had begun to grow through the protective cover and into some prints. Between 1993 and the mid-1990s the Getty Conservation Institute and Tanzania’s Department of Antiquities re-excavated the trail, removed the roots, recorded every print in detail, and reburied it under layers of sand and synthetic materials. Casts and replicas are displayed in many museums. Laetoli lies within the Ngorongoro Conservation Area, which has been a UNESCO World Heritage Site since 1979, and the site and its footprints are protected under Tanzanian law.

Myths and facts

Myth: the prints show a family walking together. Museum reconstructions, including a well-known diorama at the American Museum of Natural History in New York, have shown the Laetoli walkers as a male and female side by side, sometimes with an arm around each other. This is artistic interpretation. The prints show that the individuals walked in the same direction, probably at about the same time, but not their sex, their relationship or whether they touched.

Myth: Laetoli shows the first upright walking. Skeletal evidence from older fossils such as Ardi, about 4.4 million years old, suggests some form of upright walking had begun earlier. What makes Laetoli special is that it records the walk itself, and that the walk was already strikingly human.

Fact: the trail was nearly lost. Without the conservation work of the 1990s, tree roots would probably have destroyed many of the prints within a few decades of their excavation.

Fact: the walkers were not running. Estimates of stride length and speed suggest a slow to normal walking pace, comparable to a person strolling.

Timeline

• c. 3.66 million years ago: Hominins and animals walk across damp volcanic ash at Laetoli.
• 1935: Mary and Louis Leakey make their first visit to Laetoli.
• 1974: Mary Leakey begins a new research project at Laetoli.
• 1976: Animal footprints are discovered in the Footprint Tuff.
• 1978: Paul Abell finds the first print of the Site G hominin trail; Australopithecus afarensis is named.
• 1979: Excavation of the Site G trail is completed and published in Nature; the trail is reburied.
• 1990s: The Getty Conservation Institute and Tanzanian authorities conserve and rebury the trail.
• 2015: Footprints of two more hominins are found at Site S.
• 2021: The Site A prints are reidentified as hominin tracks.

Why it matters

Fossil bones can only hint at how an animal moved, but footprints record a moment of actual behaviour. The Laetoli trackway proved beyond reasonable doubt that hominins walked upright in an essentially human way about 3.66 million years ago, while their brains were still about the size of a chimpanzee’s. It showed that upright walking came first in human evolution, long before large brains and the first stone tools.